A conventional image reading apparatus typically has three image sensors, red (R), green (G), and blue (B), but does not have an image sensor dedicated to monochromatic images. To process monochromatic images, a monochromatic luminance signal (Y) is generated using the signals from the RGB image sensors. Generating monochromatic images in this manner results in poorer image quality due to a variety of factors, including positional differences between the three image sensors and differences between their spectral sensitivity characteristics.
More particularly, when the monochromatic luminance signal Y is generated directly from the outputs of the RGB image sensors, this can cause an inferior image quality, in part, because of differences between the original reading positions of the image sensors with respect to a pixel from which an image is being generated. Further, a CCD line sensor is typically used as a color CCD in an image reading apparatus. Its image sensors are arranged in lines with finite intervals between them, which can cause a significant decline in image quality when the monochromatic luminance signal Y is generated directly from the outputs of the image sensors without compensating for the reading position differences between the lines. Accordingly, although commonly used, CCD line sensors provide insufficient image quality for generating monochromatic images.
Furthermore, conventional image sensors have attached filters to make the image sensor highly sensitive to a specific wavelength. Such filters, however, reduce the quantity of light supplied to the image sensors. As a result, with an increased reading or scanning rate, the quantity of light supplied to each image sensor is reduced, which degrades the reading accuracy.
Although most conventional image reading devices use a color CCD with a 3-line CCD sensor corresponding to red, green, and blue, it is also possible to have a 4-line CCD sensor with a 1-line monochromatic sensor (black and white) in addition to the red, green, and blue sensors. The 1-line monochromatic sensor of the 4-line CCD sensor improves the resolution and quality of a monochromatic image by avoiding the problems of the 3-line CCD sensor, which uses a method of processing signals from two or more lines to generate monochromatic signals as explained above.
In typical 4-line CCD sensors, however, the color 1-line read frequency is one half that of the monochromatic 1-line read frequency. This difference results from the color filter coupled to each color sensor, each having one half the sensitivity as that of the monochromatic sensor.
Typically, the 4-line CCD sensor divides the 1-line input signal processing of black and white into an odd number and an even number and shares the blue process circuit when processing monochromatic signals. This sharing means that the color sensors and the monochromatic sensor cannot read signals at the same time. As a result, before reading (i.e., scanning) a document, a user must set the image reading apparatus to either a color reading setting or a monochromatic reading setting. Alternatively, a pre-scan can be performed to discriminate between a color document and a monochromatic document.
When documents have both color and monochromatic pages, several problems arise. If a color reading setting is selected, any monochromatic pages of the document suffer from the same problems that arise when using the 3-line CCD sensor, as described above, because the color sensors are used to reproduce the monochromatic image. The pre-scan approach has the drawback that valuable user time is required to perform the pre-scan.
It would be desirable for an image reading apparatus to be able to read monochromatic images rapidly and with desirable image quality, while also obtaining high quality color images.